Radio Frequency Identification (RFID) transponders (tags) are operated in conjunction with RFID base stations for a variety of inventory-control, security and other purposes. Typically an item having a tag associated with it, for example, a container with a tag placed inside it, is brought into a "read zone" established by the base station. The RFID base station generates a continuous wave electromagnetic disturbance at a carrier frequency. This disturbance is modulated to correspond to data that is to be communicated via the disturbance. The modulated disturbance, which carries information and may be referred to as a signal, communicates this information at a rate, referred to as the data rate, which is lower than the carrier frequency. The transmitted disturbance will be referred to hereinafter as a signal or field. The RFID base station transmits an interrogating RF signal which is modulated by a receiving tag in order to impart information stored within the tag to the signal. The receiving tag then transmits the modulated, answering, RF signal to the base station.
RFID tags may be active, containing their own RF transmitter, or passive, having no transmitter. Passive tags, i.e., tags that rely upon modulated back-scattering to provide a return link to an interrogating base station, may include their own power sources, such as batteries, or they may be "field-powered", whereby they obtain their operating power by rectifying an interrogating RF signal that is transmitted by a base station. Although both types of tag have minimum RF field strength read requirements, or read thresholds, in general, a field-powered passive system requires at least an order of magnitude more power in the interrogating signal than a system that employs tags having their own power sources. Because the interrogating signal must provide power to a field-powered passive tag, the read threshold for a field-powered passive tag is typically substantially higher than for an active tag. However, because field-powered passive tags do not include their own power source, they may be substantially less expensive than active tags and because they have no battery to "run down", field-powered passive tags may be more reliable in the long term than active tags. Because they do not include a battery, field-powered passive tags are typically much more "environmentally-friendly".
Such RFID systems provide significant advantages over conventional identification systems, such as barcode identification systems, in the identification, tracking, and control of objects with which they are associated. RFID systems provide a rapid read rate, whereby a number of RFID transponders, or tags, may be quickly read, outside the line-of-sight of, and at some remote location from an interrogating base station. Additionally, unlike bar codes or other write-once, read-many information storage systems, RFID tags may provide the ability of updating the information stored within the tag. Nevertheless, RFID systems would benefit from tags which are readable at as great a distance as possible.
In order to increase the operating distance of an RFID tag, one might operate at a lower carrier frequency. However, this may require a larger antenna and, therefore, a larger tag. In fact, the tag may be so big as to preclude its use in specific applications. Additionally, larger antennas may preclude the use of diversity schemes involving various positions, directions of propagation, signal polarization, and other techniques that may otherwise be employed to ensure a high "capture" or "read" rate at a given distance. Higher gain antennas may also be bulky and inconvenient for use in certain automatic identification applications. Although a brute-force approach, such as increasing the transmitting power may increase a tag's read distance, there are limits imposed, by governmental agencies, for example, on the maximum radiated power permissible. An RFID tag that maximizes read distance for a given set of power and form-factor constraints would therefore be highly advantageous.